Revert "Revert "Run exo tests in parallel""

This reverts commit b7877027.

Reason for revert: Test is still flaky after the revert.

Original change's description:
> Revert "Run exo tests in parallel"
> 
> This reverts commit 245abec2.
> 
> Reason for revert: One of the exo tests is failing flakily now. See http://crbug.com/746227
> 
> Original change's description:
> > Run exo tests in parallel
> > 
> > BUG=None
> > TEST=All tests still should pass
> > 
> > Change-Id: I8a2cc5598baaa9692a33a79767f93869e5a75b9b
> > Reviewed-on: https://chromium-review.googlesource.com/657622
> > Reviewed-by: David Reveman <reveman@chromium.org>
> > Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#500712}
> 
> TBR=reveman@chromium.org,oshima@chromium.org
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: None
> Change-Id: I6a7d7bfef56b027a0ebf8b914f2967195029840b
> Reviewed-on: https://chromium-review.googlesource.com/662842
> Reviewed-by: Ben Wells <benwells@chromium.org>
> Commit-Queue: Ben Wells <benwells@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#501215}

TBR=reveman@chromium.org,oshima@chromium.org,benwells@chromium.org

Change-Id: Ieaf5e740873c52dd9cf593cc139e493ffc7ef3c3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/663518Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501255}

components/exo/test/run_all_unittests.cc

@@ -17,7 +17,7 @@ int main(int argc, char** argv) {
   mojo::edk::Init();
 #endif
 
-  return base::LaunchUnitTestsSerially(
+  return base::LaunchUnitTests(
       argc, argv,
       base::Bind(&ash::AshTestSuite::Run, base::Unretained(&test_suite)));
 }